Batch live-stream status lookup

Filtered read returning compact live status for a batch of users (POST is used only to carry the id list; no data is written). Send a body of 1 - 50 Convex user IDs; duplicates are ignored and over-50 is rejected. Each response row is { id, isLive, platforms } and does not include the primaryStream detail returned by GET /users/{identifier}/stream. Users that do not exist, are banned, or whose profile hides stream status from the caller are omitted from the response (the batch never reveals which case applied or that a hidden user exists), so count may be less than the number of IDs sent. Unlike the single-user stream endpoint, IDs are matched by user ID only; usernames are not resolved. No pagination and no guaranteed ordering. Requires stream.read.